These 2 being my favourite DEs, I’ve got a few points of comparison:
1: Plasma is updated constantly, with tweaks and fixes with suggestions from the community taken into consideration. The KDE Devs are very responsive and constantly active. XFCE on the other hand sees updates rarely, and community suggestions tend to be ignored.
2: Plasma is a lot slicker, modern and more professional looking than XFCE. XFCE has its charm, certainly given as it’s my 2nd favourite DE, but in terms of overall presentation and day to day use, Plasma wins out in the aesthetic category.
3: Plasma has WAY more configuration options than XFCE. You can view this as a positive or negative depending on how much you like to tinker…
4: Plasma used to be a much heavier desktop experience than XFCE, which was a major plus point for XFCE, however certainly with 5.17 Plasma is as fast if not more so than XFCE on a modern system. I’ve not tried it on a resource constrained system which is where XFCE traditionally shone, but I’d have a feeling Plasma would give it a run for its money there too.
5: Plasma’s handling of panels on 2 monitors is far superior to XFCE, which for someone who has 2 monitors in work and at home is a huge plus.
6: XFCE has one advantage over Plasma: Wallpapers on virtual desktops. XFCE will allow you to set a different wallpaper per monitor (as does Plasma), but it will also allow you to set completely different wallpapers per-monitor per-virtual desktop. So if you have 2 monitors and 2 virtual desktops, XFCE will allow you to set 4 different wallpapers in total across both. Plasma has Activities where you can set differing wallpapers, but I personally don’t use them at all as I don’t really see the point of them, as I feel do a lot of other Plasma users.
For me Plasma is my favourite DE. XFCE used to be my favourite while I thought KDE was tacky and plastic looking, but after giving Plasma a proper go and trying out all the configuration and theming options, it’s now firmly overtaken XFCE as my DE of choice.
This is my current Plasma desktop at home running on Manjaro: